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Subject[PATCH 3.16 122/134] virtio_net: fix support for small rings
3.16.47-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Michael S. Tsirkin" <mst@redhat.com>

commit d85b758f72b05a774045545f24d70980e3e9aac4 upstream.

When ring size is small (<32 entries) making buffers smaller means a
full ring might not be able to hold enough buffers to fit a single large
packet.

Make sure a ring full of buffers is large enough to allow at least one
packet of max size.

Fixes: 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page frag allocators")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[bwh: Backported to 3.16:
- There's no net_device::max_mtu, so always set packet_len = IP_MAX_MTU
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/net/virtio_net.c | 30 ++++++++++++++++++++++++++----
1 file changed, 26 insertions(+), 4 deletions(-)

--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -27,6 +27,7 @@
#include <linux/slab.h>
#include <linux/cpu.h>
#include <linux/average.h>
+#include <net/route.h>

static int napi_weight = NAPI_POLL_WEIGHT;
module_param(napi_weight, int, 0444);
@@ -92,6 +93,9 @@ struct receive_queue {
/* RX: fragments + linear part + virtio header */
struct scatterlist sg[MAX_SKB_FRAGS + 2];

+ /* Min single buffer size for mergeable buffers case. */
+ unsigned int min_buf_len;
+
/* Name of this receive queue: input.$index */
char name[40];
};
@@ -600,13 +604,14 @@ static int add_recvbuf_big(struct receiv
return err;
}

-static unsigned int get_mergeable_buf_len(struct ewma *avg_pkt_len)
+static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
+ struct ewma *avg_pkt_len)
{
const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
unsigned int len;

len = hdr_len + clamp_t(unsigned int, ewma_read(avg_pkt_len),
- GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
+ rq->min_buf_len - hdr_len, PAGE_SIZE - hdr_len);
return ALIGN(len, MERGEABLE_BUFFER_ALIGN);
}

@@ -618,7 +623,7 @@ static int add_recvbuf_mergeable(struct
int err;
unsigned int len, hole;

- len = get_mergeable_buf_len(&rq->mrg_avg_pkt_len);
+ len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len);
if (unlikely(!skb_page_frag_refill(len, alloc_frag, gfp)))
return -ENOMEM;

@@ -1462,6 +1467,21 @@ static void virtnet_del_vqs(struct virtn
virtnet_free_queues(vi);
}

+/* How large should a single buffer be so a queue full of these can fit at
+ * least one full packet?
+ * Logic below assumes the mergeable buffer header is used.
+ */
+static unsigned int mergeable_min_buf_len(struct virtnet_info *vi, struct virtqueue *vq)
+{
+ const unsigned int hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
+ unsigned int rq_size = virtqueue_get_vring_size(vq);
+ unsigned int packet_len = IP_MAX_MTU;
+ unsigned int buf_len = hdr_len + ETH_HLEN + VLAN_HLEN + packet_len;
+ unsigned int min_buf_len = DIV_ROUND_UP(buf_len, rq_size);
+
+ return max(min_buf_len, hdr_len);
+}
+
static int virtnet_find_vqs(struct virtnet_info *vi)
{
vq_callback_t **callbacks;
@@ -1517,6 +1537,7 @@ static int virtnet_find_vqs(struct virtn

for (i = 0; i < vi->max_queue_pairs; i++) {
vi->rq[i].vq = vqs[rxq2vq(i)];
+ vi->rq[i].min_buf_len = mergeable_min_buf_len(vi, vi->rq[i].vq);
vi->sq[i].vq = vqs[txq2vq(i)];
}

@@ -1601,7 +1622,8 @@ static ssize_t mergeable_rx_buffer_size_

BUG_ON(queue_index >= vi->max_queue_pairs);
avg = &vi->rq[queue_index].mrg_avg_pkt_len;
- return sprintf(buf, "%u\n", get_mergeable_buf_len(avg));
+ return sprintf(buf, "%u\n",
+ get_mergeable_buf_len(&vi->rq[queue_index], avg));
}

static struct rx_queue_attribute mergeable_rx_buffer_size_attribute =
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